Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and other content. These systems may be multiple-access systems capable of simultaneously supporting communication for multiple wireless terminals by sharing the available transmission resources (e.g., frequency channel and/or time interval). Since the transmission resources are shared, efficient allocation of the transmission resources is important as it impacts the utilization of the transmission resources and the quality of service perceived by individual terminal users. One such wireless communications system is the Orthogonal Frequency-Division Multiple Access (OFDMA) system in which multiple wireless terminals perform multiple-access using Orthogonal Frequency-Division Multiplexing (OFDM).
OFDM is a multi-carrier modulation technique that partitions the overall system bandwidth into multiple orthogonal frequency subchannels, each of which is associated with a respective subcarrier that may be modulated with data. Because the subchannels are made orthogonal, some spectral overlap between the subchannels is permitted, leading to a high spectral efficiency. In OFDM systems, the user data stream is split into parallel streams of reduced rate, and each obtained substream then modulates a separate subcarrier.
In OFDMA, access to the shared wireless medium is scheduled using frames that extend over two dimensions: time, in units of symbols, and frequency, in units of logical sub-channels. Data bursts are conveyed in two-dimensional (i.e. time and frequency) data regions within the frame which are scheduled by the BS via specific control messages. Each frame is divided into downlink (DL) and uplink (UL) subframes. The former is used by the BS to transmit data to the MSs, whereas the MSs transmit to the BS in the latter.
Examples of OFDM communication systems include, but are not limited to, wireless protocols such as the wireless local area network (“WLAN”) protocol defined according to the Institute of Electrical and Electronics Engineering (“IEEE”) standards radio 802.11a, b, g, and n (hereinafter “Wi-Fi”), the Wireless MAN/Fixed broadband wireless access (“BWA”) standard defined according to IEEE 802.16 (hereinafter “WiMAX”), the mobile broadband 3GPP Long Term Evolution (“LTE”) protocol having air interface High Speed OFDM Packet Access (“HSOPA”) or Evolved UMTS Terrestrial Radio Access (“E-UTRA”), the 3GPP2 Ultra Mobile Broadband (“UMB”) protocol, digital radio systems Digital Audio Broadcasting (“DAB”) protocol, Hybrid Digital (“HD”) Radio, the terrestrial digital TV system Digital Video Broadcasting-Terrestrial (“DVB-T”), the cellular communication systems Flash-OFDM, etc. Wired protocols using OFDM techniques include Asymmetric Digital Subscriber Line (“ADSL”) and Very High Bitrate Digital Subscriber Line (“VDSL”) broadband access, Power line communication (“PLC”) including Broadband over Power Lines (“BPL”), and Multimedia over Coax Alliance (“MoCA”) home networking.
3GPP LTE defines the following physical channels:                Downlink (DL)                    Physical Broadcast Channel (PBCH): This channel carries system information for mobile stations (referred to as user equipment, or UE) requiring access to the network.            Physical Downlink Control Channel (PDCCH): The main purpose of this physical channel is to carry scheduling information.            Physical Hybrid ARQ Indicator Channel (PHICH): This channel is used to report the Hybrid ARQ status.            Physical Downlink Shared Channel (PDSCH): This channel is used for unicast and paging functions.            Physical Multicast Channel (PMCH): This physical channel carries system information for multicast purposes.            Physical Control Format Indicator Channel (PCFICH): This channel provides information to enable the UEs to decode the PDSCH.                        Uplink (UL)                    Physical Uplink Control Channel (PUCCH): This channel is used to transport user signalling data from one or more UE that can transmit on the control channel. The PUCCH transports, for example, acknowledgment responses and retransmission requests, service scheduling requests, and channel quality information measured by the UE to the system.            Physical Uplink Shared Channel (PUSCH): This channel is used to transport user data from one or more mobiles that can transmit on the shared channel.            Physical Random Access Channel (PRACH): This uplink physical channel allows a UE to randomly transmit access requests when the UE attempts to access the wireless communication system.                        
Wireless communication systems may employ a relay scheme to relay user data and possibly control information between a base station (BS) and a mobile station (MS) through one or more relay stations (RS). A relay scheme may be used to enhance coverage, range, throughput and/or capacity of a base station. The relay stations can repeat transmissions to/from the BS so that MSs within communication range of a relay can communicate with the BS through the relay. The relays do not need a backhaul link because they can communicate wirelessly with both BSs and MSs. This type of network may be referred to as a multihop network because there may be more than one wireless connection between the MS and a hardwired connection. Depending upon the particular network configuration, a particular MS may gain network access via one or more neighbour relays and/or one or more neighbour BSs. In addition, relays themselves might have one or more available path options to connect to a particular BS. The radio link between a BS or RS and an MS is called an access link, while the link between a BS and an RS or between a pair of RSs is called a relay link.
Conventional relays operate in one of two different modes: transparent and non-transparent. A transparent RS does not transmit control information, such that a MS connected to a transparent RS receives control information directly from the BS, and the RS relays only data traffic. A non-transparent RS transmits control information and relays data traffic as well.
Hybrid automatic repeat-request (HARQ) operations can be used for error control in wireless communication systems. With HARQ, the receiver detects an error in a message and automatically requests a retransmission of the message from the transmitter. In response to receiving the HARQ request (a “NACK”), the transmitter retransmits the message until it is received correctly, unless the error persists. In one variation, HARQ combines forward error correction (FEC) with an error-correction code.
LTE uses asynchronous HARQ transmission on the DL. In asynchronous HARQ, the receiver does not know ahead of time when the retransmission is being sent, and therefore control information must be sent along with the data. This is accomplished by sending resource allocation messages on the PDCCH simultaneous to the corresponding PDSCH transmission. The advantage of this scheme is that the scheduling algorithm has considerable freedom in deciding which MSs are sent data during any subframe.
In LTE systems where transparent relays are used, a RS could help improve system performance by sending DL HARQ retransmissions to the MS at the same time as the BS. However, an issue arises as to how the BS and the RS can coordinate concurrent DL HARQ retransmission. Prior to retransmission, the RS has to know which physical resources (time and frequency) are used for retransmission of the packet by the BS so that the RS can use the same resources to transmit the same packet concurrently. However, since DL HARQ retransmissions are asynchronous, the BS sends PDCCH and PDSCH in one subframe for retransmission when a NACK is received. As the control signalling region and data transmission region are multiplexed contiguously in time division multiplexing (TDM) fashion, there is no guard time between the two regions. The PDCCH is transmitted in the first n (where n=1, 2 or 3) OFDM symbols in each subframe, and the PDSCH is transmitted through the remaining (N−n) OFDM symbols (where N is the number of OFDM symbols in each subframe). It is difficult for the RS to switch from reception mode to transmission mode between contiguous symbols. It is also difficult for the RS to both decode retransmission control information in the PDCCH and prepare retransmission in the PDSCH in the same subframe. Additionally, in some situations the number of PDCCH carried by PCFICH could vary from subframe to subframe, requiring the RS to decode PCFICH, determine the start of PDCCH and prepare retransmission in the PDSCH in the same subframe.
While use of synchronous HARQ (i.e. retransmissions are scheduled on predetermined subframes) might alleviate some of the aforementioned difficulties, such an approach could introduce undesirable restrictions on the scheduler.
A need exists for an improved scheme for downlink retransmissions in transparent relay systems.